Cerebral palsy (CP) has a prevalence of 1.5-3 per 1000 live births, making it the most common neurological disorder among children in Europe. One of the most frequently observed problems in CP is hypertonia, i.e. increased muscle tension, which can result in contractures. Stretching therapies are widely used as a treatment for contractures, but with highly variable success. Therefore, efforts are needed to improve the efficacy of stretching interventions. The investigators hypothesise that increasing the stiffness of the tendon relative to the muscle, by resistance training, prior to stretching will improve the efficacy of stretching interventions in children with CP. In a RCT design the investigators will compare a group of children receiving a combined strengthening-stretching intervention of the calf muscle to a control group receiving conventional stretching exercises in combination with upper limb exercises. Outcome measures will include muscle-tendon structure and gait analysis to assess functional improvements. This research will improve the understanding of muscle responses to stretching interventions in children with CP and can lead to more effective stretching therapies.